The present invention relates generally to the detection and recording of nuclear particles. More particularly, the present invention relates to a system for detecting and recording the impingement of a nuclear particle on a two-dimensional position-sensitive solid-state detector.
One of the simplest and well known prior art devices useful for detecting and recording the presence of a nuclear particle or radiation image is photographic film which uses a silver halide. However, such materials are difficult to use under various adverse conditions, have a limited lifetime and can be generally difficult to work with. Furthermore, such materials cannot provide a precise position detection function.
The detection of nuclear particle impingement together with position sensing over relatively large areas has, in the past, generally required rather expensive two-dimensional arrays of detectors. The use of such two-dimensional arrays also requires considerably complex position-sensing circuitry for use with such arrays in order to achieve good resolution of the impingement location.
Consequently, a wide range of detectors has been developed which provide benefits over the use of detector arrays. Such detectors are generally semiconductor devices in which the radiation absorption and detection occur in the same material, for example, silicon or germanium diodes, or scintillator media mated with photo-multiplier tubes in which the configuration of light produced in the scintillator is detected by the photo-multiplier tube. Radiographic films have the additional drawback that they do not allow for easy retrieval of the stored information relating to the detected impingement of the radiation particles.
In the past, semiconductor detectors have generally been the most useful for particulate radiation, because the range of the particles is usually less than the depletion region depth of the detectors. Such semiconductor detectors have good energy resolution, excellent timing characteristics, good stability and simplicity of operation. However, the area of such detectors has been limited to approximately 20 cm.sup.2 or less. Thus, the necessity for arraying individual detectors.
Various other methods for recording and reproducing radiation images are also known. For example, British Patent Application No. 1,462,769 and Japanese Patent Laid Open No. 29,889/1976 disclose a method in which a stimulable phosphor absorbs radiation passing through an object. The phosphor can then be stimulated using a certain kind of energy to release the radiation stored in the phosphor as luminescence which can then be detected. Both the British patent and Japanese Laid Open patent disclose a method which is a subset of that method, namely, the use of a heat-stimulable phosphor and thermal energy as the storing medium and releasing energy, respectively.
The method disclosed in those patents involves the use of a thermoluminescent phosphor layer provided on a support base for recording the impingement of radiation thereon. By heating the support base and thermoluminescent phosphor layer, the radiation energy corresponding to the radiation impingement is released as a light signal which can then be detected. However, a major drawback of that type of detector is that the use of a heat-stimulable phosphor requires that the panel or support base upon which the thermoluminescent phosphor is placed be extremely heat resistant. Furthermore, because of the necessity for heating the detector to a high temperature, local heating conditions result in poor position resolution with respect to radiation impingement. Also, the response time of such a detector is very slow due to the thermal time constants involved. In addition, the use of a CO.sub.2 laser limits the position resolution achievable to 10 microns or greater.
U.S. Pat. No. 3,859,527 discloses an apparatus which does not utilize the input of heat energy in order to release the stored radiation pattern. The stimulable phosphor utilized in that patent is a visible ray or infrared ray stimulable phosphor and visible rays or infrared rays are utilized as the stimulation energy. Visible rays or infrared rays are used in order to convert the radiation energy stored in the apparatus to a light signal. However, there are only a limited number of phosphors, such as cerium and samarium activated strontium sulfide, a europium and samarium activated strontium sulfide, a europium and samarium activated lanthanum oxysulfide phosphor and a manganese and halogen activated zinc cadmium sulfide phosphor which can be used. The sensitivity of the method disclosed in the '527 patent and the phosphors employed therein is very low because the stimulability of such phosphors is very low. Thus, the method and apparatus disclosed in the '527 patent is of little practical use. Contrary to the statement at column 4, lines 11-12 of that patent, applicants have found that the cerium and samarium activated strontium sulfide composition is sensitive to tungsten light.
U.S. Pat. No. 4,239,968 discloses yet another method and apparatus for recording and reproducing a radiation image in which the radiation image is recorded on a stimulable phosphor and the recorded image is reproduced by utilizing the stimulability of the phosphor. The phosphor disclosed for use in that patent is an alkaline earth metal fluorohalide phosphor which is claimed to have high stimulability. The phosphor disclosed in that patent is represented by the formula (Ba.sub.l-x M.sub.x.sup.II) FX:yA, in which M.sup.II is at least one divalent metal selected from the group consisting of Mg, Ca, Sr, Zn and Cd, X is at least one halogen selected from the group consisting of Cl, Br and I, A is at least one element selected from the group consisting of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb and Er, preferably selected from the group consisting of Eu, Tb, Ce and Tm and x and y are numbers satisfying the conditions of 0 is less than or equal to x is less than or equal to 0.6 and 0 is less than or equal to y which is less than or equal to 0.2.
The method described in the '968 patent comprises the steps of causing a visible ray or infrared ray-stimulable phosphor to absorb radiation passing through an object, stimulating the phosphor by stimulation rays selected from visible rays and infrared rays in order to release the energy of the radiation stored therein as luminescence and detecting the luminescence. The stimulation rays are described as having a wavelength of not less than 500 nm. The apparatus utilized for such recording and reproducing of a radiation image is comprised of a radiation image storage panel using a stimulable phosphor which absorbs radiation passing through an object, a light source emitting stimulation rays of not less than 500 nm in order to release the energy of the radiation stored in the phosphor as luminescence and a detector for detecting that luminescence. In both the method and apparatus aspects, the stimulable phosphor is at least one phosphor selected from the group of the above-mentioned alkaline earth metal fluorohalide phosphors.
However, the method and apparatus set forth in the '968 patent has many drawbacks. First, the intensity of the stimulating radiation is very high. Secondly, the efficiency of the phosphor disclosed therein is not efficient enough to be utilized as a relatively large area particle detector because of the sensitivity of the phosphors disclosed therein with regard to the area to be covered. A further drawback of the method and apparatus disclosed in the '968 patent is that it cannot be utilized with readily available low cost infrared sources, for example, laser diodes, since utilization of halide/phosphors require helium-neon ruby lasers. Another drawback to the method and apparatus disclosed in the '968 patent is that it has been found in use that the depth of the traps of the materials disclosed therein is insufficient for the storage of energy over a long period of time. Still another drawback to the '968 device is that it cannot readily utilize infrared wavelengths for both stimulation and emission since good separation between the stimulating and emitted wavelengths is required.
The present invention is directed to detecting impingement of nuclear particles with position sensing over large areas in which the detector provides instantaneous local scintillation at visible wavelengths upon nuclear particle impingement. In addition, storage of a trap charge at such locations is also provided which can later be interrogated and read by the application of near-infrared illumination. Such illumination may be readily provided by solid state sources such as infrared-emitting diodes or laser diodes. The solid-state materials utilized by the present invention are a family of materials known as alkaline earth sulfides and selenides that have highly unusual properties. These particular materials display efficient electron trapping and are called electron trapping materials. These materials can be "charged" by light. That is, upon energetic illumination, electrons are raised to a higher trapping state where they stay indefinitely. Upon arrival of low energy photons, such as contained in an infrared beam, these materials can emit an orange or blue light through the release of the trapped electrons.
In the 1930's and 1940's, natural thermoluminescence was observed in ordinary limestones and granites. While that effect could only be observed once, the emission properties of the limestones and granites could be restored by irradiation by x-rays or gamma rays. The effect was studied in detail and reported by F. Daniels, et al, "The Thermoluminescence of Crystals", Report On Contract AT(11-1) -27, University of Wisconsin, Madison, Wis. A general model involving trapped electrons was later accepted. See, "Electronic Processes in Ionic Crystals", N. F. Mott and R. W. Gurney, Oxford University Press, New York, 1940. Unlike that early work and the work disclosed in the British and Japanese patent applications described above, the method and apparatus of the present invention releases the trapped electrons by use of infrared or near-infrared radiation. In order to measure an accumulated dose of high energy radiation, rather than heating the sample, the dose can be "read" by, for example, infrared pulses. Such pulses will release the trapped electrons and supply an analog of the integrated dose for the previous time interval in the form of the integral released intensity of orange or blue light.
Since the state of the sensor described herein can be tested by an electromagnetic wave, which then supplies information in the form of yet another electromagnetic wave, the device described herein can be termed a photonic detector. The present application is directed to the use of such material to large areas which can be used in many different environments as a nuclear particle detector and event locator with memory.
In order to more clearly describe the family of materials which are utilized by the instant invention, it is believed useful to review the history and terms utilized in this field. The basis for this discussion is the term luminescence, which is understood to be the ability of certain solids to emit light under different conditions.
While luminescence is a long known phenomenon of nature which was first observed at least as early as the 19th century, research in this area seemed to cease with the end of World War II. While much of the early work performed by Seebeck, Becquerel, Leonard, Charbonneau and Urbach demonstrated the existence of the phenomenon and attempted to apply it to practical use, the sensitivities of the known phosphors at that time were far too low for practical use.
After World War II, extensive work continued in the area of cathodoluminescence for use with the screens of cathode ray tubes, and other applications which involve the down conversion of photon energy, rather than the up conversion utilized by the present invention. It is relatively easy to produce the instantaneous emission of light from solids utilizing excessive incoming photon or particle energy, such as accelerated electrons, ultraviolet radiation, x-rays, etc. However, the use of up converting materials, such as infrared phosphors, has been practically nonexistent over the last forty years.
Due to the limited amount of work with phosphors and the time period over which such work was performed, the state of the infrared phosphor field is primitive indeed. Only minimal theories explaining the operation of such materials and little working technology existed prior to the work by the assignee of the present invention. It should be remembered that in the early 1940's, even the electronic structure of materials was still not fully understood, although the concept of band gap was introduced just a few years earlier and Schottky was explaining the electrical characteristics of metal-semiconductor junctions. Schottky, et al. discovered the transistor in 1949 and the theories and applications of solid state materials took an explosive course. Materials with traps, however, were not part of this revolution and, in fact, a whole generation of semiconductor physicists regarded trapping effects as a highly undesirable phenomenon and continuously worked to keep this disturbing effect from influencing their semiconductor devices.
While luminescence refers to the ability of certain solids and liquids to emit light when driven by an external energy source, when the driving energy source is light, the proper term for that phenomenon is photoluminescence. While certain photoluminescent materials will emit light when driven by radiation of short wavelengths, such as ultraviolet, there is another class of materials, which, upon excitation by illumination, can store electrons in "traps" for extremely long time periods. The trapped electrons can later be released, upon command, by less energetic photons. If the emission of visible light is activated by, for example, infrared illumination, a photon energy up conversion takes place because energy is stored in the material. The present invention relates to that class of materials with the unique properties of electron traps. Utilizing such optical up conversion from one wavelength to another, optical communications can be accomplished, as well as making infrared radiation patterns visible. In further applications, in a light-to-light photonics mode, such materials will be used in digital processing or so-called "light" computers.